The present invention relates to a circuit arrangement including at least one delayed-action semiconductor switch and an auxiliary circuit for influencing the so-called storage period, which is the delay occurring during the switching from the conductive (switch-on phase) to the blocking (blocking phase) state of a semiconductor switch. More particularly, the present invention relates to such a circuit arrangement wherein the auxiliary circuit has a control path extending from a tap in the load current path leading through the switching path of the semiconductor switch to a circuit point in the control signal path leading to the control electrode of the semiconductor switch, and wherein an actual value, is when there is a change in the storage period, which is obtained at the tap and is used to shorten the storage period by influencing a control value which is fed to the control electrode of the semiconductor switch during the switch-on phase.
A circuit arrangement as described above is disclosed in connection with FIG. 2 of German Offenlegungsschrift (Laid-open Patent Application) No. P 27 15 133 of Oct. 12th, 1978. FIG. 1 of the present application shows a portion of the circuit of the above cited Patent Application. Insofar as it is of interest here, the Patent Application states:
" . . . the positively actuated switching stage . . . includes a switching transistor Ts6/9 and a PDM amplifier connected ahead of it . . . . In order to avoid storage times, the transistors must not be driven into saturation." A switching transistor Ts5 is connected as emitter-follower. "Without PDM signal the transistor Ts1 is blocked. Thus, the constant current source Ts2 is blocked as well. Ts3 is conductive because it receives its base current via resistors R5 and R6 and diodes Gr5 and Gr6. A diode Gr3, by base current limitation, prevents the transistor Ts3 from being driven into saturation. This is done in that, upon a drop in the collector-emitter voltage of the transistor Ts3 to approximately 1.5 V or less, current flows through the diode Gr3 to the collector. In this state the voltage drop from the reference point 28 remote from the base to the emitter is a combination of the threshold voltages of the diodes Gr5, Gr6 and the base-emitter diode. This is opposed by the voltage drop from point 28 via a collector input lead point 29 to the emitter. This voltage drop is a combination of the threshold voltage of diode Gr3 and the collector-emitter voltage which must not drop to the saturation voltage . . . . When a positive PDM pulse arrives at the terminal 27, Ts1 becomes conductive and switches on . . . the constant current source including transistor Ts2 . . . Ts3 is . . . blocked. After blockage . . . the emitter-follower Ts4 as well as the switching transistors Ts5 and Ts6/9 become conductive . . . . When the collector-emitter voltage of the switching transistors has dropped to about 6 V, a diode Gr8 becomes conductive. By means of base current limitation in Ts4 and Ts5 with the aid of diode Gr8, the switching transistor are prevented from being driven further into saturation . . . .
Particularly notable . . . are the measures for shortening the switching times which make it possible to . . . keep the power losses low."
The base current limitation for transistors Ts1 or Ts4, Ts5, respectively, by means of diodes Gr3 or Gr8, is based on the consideration that the storage period for a switching transistor, i.e. the switching delay during switching from the conductive to the blocking state, is particularly large if the transistor is driven into current saturation when it is in the forward conducting state. During switching into the blocking state the charge carriers can then not be transported away quickly enough. Therefore, care must be taken that current saturation will not be reached when the transistor is in the forward conducting state. This is possible by limiting the base current.
A criterion for the limitation of the base current by conducting away part of the current via diodes Gr3 or Gr8, respectively, to the associated collector of a switching transistor is the drop of the collector-emitter voltage to a value which is characteristic for the approximation to current saturation. Then the potential across the collector drops to such an extent that the above-mentioned diodes become conductive. Thus, the beginning base current limitation during a switch-on phase will significantly shorten the subsequent storage period.
It has now been found in some cases that the shortening of the storage period is insufficient and that in spite of short storage periods there may still occur operating malfunctions if the switching time is of any significance at all. This is the case, for example, when simultaneously switched transistors are connected in series. Experiments have shown that the collector-emitter voltages of series connected switching transistors in blocking state are divided up in very different amounts if their storage periods differ by only 25 ns. This is understandable if it is considered that when one transistor is blocked the other transistor is no longer able to reduce its free charge carriers by means of more collector current. In this connection, it is customary to overcome this problem by obtaining a uniform voltage distribution in that the switching transistors are connected in parallel with a chain of components which may include resistors, diodes and capacitors. These measures are complicated and lead to an increase in power losses.
Even in a switching amplifier (driver) for controlling a switching tube in a high power pulse duration modulator (PDM) it has been found that in spite of very short storage periods there may occur signal distortions (pulse distortions) and destruction of the switching transistors, particularly if the switching transistors are operated in series and/or in push-pull connections.
Signal distortions are noted in particular if the current to be switched through by the switching transistor is not constant but is variable in the sense of an amplitude comodulation of the pulse duration modulated pulse, i.e. if the load current--in a transistor this is the collector-emitter current--of the switching transistor is variable.